1. Technical Field
The present invention relates to a pulse width modulation (PWM) drive control of a multi-phase electric motor such as a three-phase brushless motor. In particular, the present invention relates to current detection in a controller arranged with a single current detector between a drive circuit for PWM driving and a direct current (DC) power supply (high voltage side or low voltage side).
2. Related Art
A PWM signal is generated by comparing a carrier wave of saw-tooth shape or triangular shape (saw-tooth signal, triangular signal) and a duty set value corresponding to a target current value in each phase of the multi-phase electric motor. That is, whether the PWM signal is high level or low level is determined depending on whether a value (value of PWM counter) of the saw-tooth signal or the triangular signal is greater than or equal to, or smaller than a duty set value.
When the current detector attempts to measure a current value at a predetermined time in the controller of the multi-phase electric motor for generating the PWM signal based on the saw-tooth signal or the triangular signal and driving the multi-phase electric motor, a time interval in time of switching between one phase and another phase sometimes becomes very small. Since the current is not stable due to the switching time of an electric field effect transistor of the drive circuit, the presence of dead zone (dead time), and also the response delay of an electronic processing circuit, an accurate current cannot be measured during such period.
For instance, when using an A/D converter for the current detector, an accurate current value cannot be detected unless a stable signal is continuously inputted for at least 2 μs due to the specification of the A/D converter. If the input signal is not stably inputted continuously for 2 μs, the A/D converter cannot detect an accurate current value of each phase.
In a vehicle steering device described in Japanese Patent Application Laid-Open Publication No. 2007-112416, a single current sensor for detecting the current value flowing through a current path is arranged on the current path between a motor drive circuit and a ground, and a phase of a saw-tooth wave for generating the PWM signal of each phase is shifted to shift the timing of fall of the PWM signal of each phase to the low level. A value of a U-phase current flowing through the electric motor is then obtained based on an output signal of the current sensor during a period in which a predetermined time has elapsed from when the PWM signal of the V phase fell to the low level. A total current value of the U-phase current and the V-phase current flowing through the electric motor is obtained based on an output signal of the current sensor during a period in which a predetermined time has elapsed from when the PWM signal of the W phase fell to the low level.
In the method of controlling a three-phase or multi-phase inverter described in Japanese Laid-Open Patent Publication No. 10-155278, if the time interval between the switching of a transistor of one phase and the switching of a corresponding transistor of the next phase is smaller than a predetermined threshold value within the PWM period, the measurement is prohibited, the PWM signal defining the measurement time interval of sufficient duration is generated, and the measurement of the influence of switching on a line current becomes possible. The duration of the other PWM signals of the same dependent period is reduced by a certain value, and the sum of reduction of such other PWM signals is obtained to compensate for the amount of increase of the PWM signal defining the measurement interval.
A drive system for a three-phase brushless AC motor described in Japanese Laid-Open Patent Publication No. 2005-531270 is configured to optimize a transistor switching pattern to enhance the power output while enabling the measurement of the current in all phases using a single sensor. This is realized by defining a voltage demand vector x in a case where three or more states are required to satisfy the minimum state time requirement determined by the single sensor method, and calculating the three or more state vectors for generating the request vector x while still allowing the single current detection.
In a method of monitoring a brushless motor capable of compensating some kind of drift in the output signal during the motor operation described in Japanese Laid-Open Patent Publication No. 2001-95279, the current flowing into or flowing out from each winding of the motor is monitored using a current measurement section and an output signal displaying the current is generated, the output of the current measurement section is measured when an instantaneous current flowing through the current measurement section is known to be substantially zero, and a correction output signal for compensating some kind of difference between an actual measurement output signal value and an ideal output signal value is generated.
In U.S. Pat. No. 6,735,537, the triangular signal is used for the carrier wave, terms h phase, m phase, and l phase are used in place of the terms U phase, V phase, and W phase, and the time interval between the h phase and the m phase is represented as t1 and the time interval between the m phase and the l phase is represented as t2. As shown in FIG. 7 of U.S. Pat. No. 6,735,537, the process of Case 2 is performed when the time intervals t1, t2 are both smaller than a threshold value (mw). The process of Case 3 or Case 4 is performed when either one of the time intervals t1, t2 is smaller than the threshold value (mw). In a case of the process of Case 2 (see FIG. 13), the Duty maximum phase is shifted to the left side, and the Duty minimum phase is shifted to the right side (see FIG. 12B). If in a case of the process of Case 3 (see FIG. 15), and determined that only one phase needs to be shifted (N of step 148), the Duty maximum phase is shifted to the left side (see FIG. 14B). If in a case of the process of Case 4 (see FIG. 17), and determined that only one phase needs to be shifted (N of step 166), the Duty minimum phase is shifted to the left side (see FIG. 16B).